In association of the lac repressor with its operator is the best understood example of a specific protein-DNA recognition process. Our studies have been directed towards providing a dynamic picture of the events which transpire as this protein binds to DNA. We have extensively studied the interaction of this protein with nonoperator DNA. Most of these studies have relied on fluorescent proves attached to the repressor to monitor its association with DNA. Stopped-flow rapid kinetic studies have revealed several conformational changes occurring within the repressor as it binds to the nonoperator DNA, poly(d(A-T)). These studies will be repeated using a 40 base pair DNA fragment containing the lac operator sequence. By comparing the conformational changes within the repressor as it binds to both operator and nonoperator DNA, we hope to reveal differences which are responsible for the selectivity of this protein for its operator. We will also carry out stopped-flow fluorescence polarization studies and ionic strength jump relaxation studies to determine if these conformational changes occur before or after the repressor binds to nonoperator DNA.